(Reuters) - The U.S. Supreme Court signaled a willingness on Monday to rein in President Barack Obama's power to temporarily fill senior government posts without the Senate's approval, a move that would curb his ability to bypass a gridlocked Congress.

Most of the nine justices expressed skepticism, during 90 minutes of oral arguments, about so-called recess appointments Obama made to the National Labor Relations Board (NLRB) in 2012.

The court is expected to issue a ruling in the case by June that has the potential to shift the balance of power between the White House and the Senate. While both are now controlled by Democrats, Republicans hope to win control of the Senate in congressional mid-term elections in November.

The Supreme Court could decide the case in various ways, but even a narrow ruling against the administration could be bad news for Obama in the last two years of his term, especially if Republicans control the Senate.

The arguments before the court on Monday dealt with a case in which soft drink bottler Noel Canning Corp is challenging an NLRB ruling against it. The company argues the ruling was invalid because some of the NLRB board members on the panel that issued it were recess appointees picked by Obama.

With the intervention of senior Republicans, the U.S. Chamber of Commerce and other business interests, the Yakima, Washington-based company's case has become a much broader fight over the president's ability to make appointments while the Senate is in recess and what exactly constitutes a recess.

Obama used his recess appointment power to name three members to the five-member NLRB in January 2012. Democratic and Republican presidents have made many such temporary appointments - valid for up to two years - of officials who otherwise would have had difficulty winning Senate confirmation.

Underscoring the political stakes involved in the court case, Senate Republican Leader Mitch McConnell, who has sought to stymie Obama's legislative agenda, attended the arguments.

Under McConnell, Republicans had used Senate rules to frustrate Obama's attempts to fill various positions, including vacancies at the NLRB, up until a Senate rule change was pushed through by Democrats late last year.

"The president made an unprecedented power grab by placing political allies at a powerful federal agency while the Senate was meeting regularly and without even trying to obtain its advice and consent," McConnell said in a statement afterward.

White House Counsel Kathryn Ruemmler and White House spokesman Jay Carney were also present. Carney told reporters the administration was "confident that the president's authority to make recess appointments will be upheld by the courts."

FRAMERS' INTENT DEBATED

The administration says it is following the long-established interpretation of the recess appointments clause of the U.S. Constitution, dating back to President George Washington.

Noel Canning and its backers say the administration has ignored the original intent of the Constitution's drafters, who included the recess appointments clause to ensure the government could continue to function when the Senate was in recess for months at a time and senators would travel to Washington on horseback.

The court could decide the case in various ways, but even a narrow ruling against the government could be bad news for Obama in the last two years of his term.

If Republicans win control of the Senate in November, they would be able to reject Obama appointments outright and would have more sway over when to declare recesses.

Republicans and business groups like the U.S. Chamber of Commerce have long been eager to prevent Democratic presidents from appointing pro-labor members to the NLRB, an independent federal agency which has the power to address unfair labor practices and safeguard employees' rights.

These groups were particularly outraged at Obama naming the three NLRB members while the Senate was not conducting business, but was not technically in a recess.

POLITICAL DISPUTE

At least one justice appeared to see the dispute as primarily political.

Justice Stephen Breyer, an appointee of Democratic President Bill Clinton, said the Constitution clearly envisioned that appointments had to be agreed upon by both the president and Congress. "Now that's a political problem, not a constitutional problem" if the two sides disagree, he said.

Despite apparent misgivings about whether the courts should be deciding such an issue, justices from both sides of the ideological divide expressed skepticism about the administration's use of the recess appointment power.

Justice Elena Kagan, appointed to the court by Obama in 2010, was one of those critical of Solicitor General Donald Verrilli's legal arguments on the administration's behalf. The administration, for example, says that it is up to the president to determine when exactly the Senate is in recess.

But, to Kagan, some of Verrilli's arguments seemed to confirm that "it's really the Senate's job to determine whether they're in recess," she said.

Chief Justice John Roberts defended the right of senators to object to appointments they do not like. "They have an absolute right not to confirm nominees that the president submits," he said.

"EQUILIBRIUM" CITED

Verrilli's defense was that the way the recess appointments clause has been used over the years has changed in an effort to create a "stable equilibrium" between executive branch and congressional power.

If a majority of justices were to buy that argument, the administration could still lose on a narrower ground, but the recess appointment power would not be entirely disabled. Monday's argument indicated that, although there could be a clear majority to rule against the administration on that narrow ground, some justices might be willing to go further.

If Noel Canning wins its case, the practical fallout for the NLRB would be limited. In July, a Senate deal paved the way for the confirmation of five board members, marking the first time in a decade that the board had a full complement.

If the bottler prevails, those members would have to re-examine some board decisions made by the contested recess appointees.